Turbochargers are a type of forced induction system. They deliver air, at greater density than would be possible in the normally aspirated configuration, to the engine intake, allowing more fuel to be combusted, thus boosting the engine's horsepower without significantly increasing engine weight. This can enable the use of a smaller turbocharged engine, replacing a normally aspirated engine of a larger physical size, thus reducing the mass and aerodynamic frontal area of the vehicle.
Turbochargers (FIG. 1) use the exhaust flow (100), which enters the turbine housing at the turbine inlet (5) of a turbine housing (2), from the engine exhaust manifold to drive a turbine wheel (70), which is located in the turbine housing. The turbine wheel is solidly affixed to one end of a shaft, the other end of which contains a compressor wheel (20) which is mounted to the shaft and held in position by the clamp load from a compressor nut. The primary function of the turbine wheel is providing rotational power to drive the compressor. Once the exhaust gas has passed through the turbine wheel (70) and the turbine wheel has extracted energy from the exhaust gas, the spent exhaust gas (101) exits the turbine housing (2) through the exducer (6) and is ducted to the vehicle downpipe and usually to after-treatment devices such as catalytic converters, particulate traps and NOx traps.
The power developed by the turbine stage is a function of the expansion ratio across the turbine stage, which is the expansion ratio from the turbine inlet (5) to the turbine exducer (6). The range of the turbine power is a function of, among other parameters, the flow through the turbine stage.
The compressor stage consists of a wheel and its housing. Filtered air is drawn axially into the inlet (11) of a compressor cover (10) by the rotation of the compressor wheel (20). The power generated by the turbine stage to the shaft and wheel drives the compressor wheel (20) to produce a combination of static pressure with some residual kinetic energy and heat. The pressurized gas exits the compressor cover (10) through the compressor discharge (12) and is delivered, usually via an intercooler, to the engine intake.
The design of the turbine stage is a compromise among: the power required to drive the compressor at different flow regimes in the engine operating envelope; the aerodynamic design of the stage; the inertia of the rotating assembly, of which the turbine is a large part, since the turbine wheel is manufactured typically in Inconel, which has a density 3 times that of the aluminum of the compressor wheel; the turbocharger operating cycle, which affects the structural and material aspects of the design; and the near field both upstream and downstream of the turbine wheel with respect to blade excitation.
The basic turbocharger configuration is that of a fixed turbine housing. In this configuration, the shape and volume of the turbine housing volute is determined at the design stage and cast in place. The basic fixed turbine housing is the most cost-effective of the following options simply because it is the most simple and has the fewest parts.
The next level of sophistication is that of a wastegated turbine housing. In this configuration, the volute is cast in place, as in the fixed configuration above. The volute (21) is fluidly connected to the exducer (6) by a duct. Flow through the duct is controlled by a wastegate valve. Because the outlet of the wastegate duct is on the exducer side of the volute, which is downstream of the turbine wheel, flow through the wastegate duct, when in the bypass mode, bypasses the turbine wheel (70), thus not contributing to the power delivered to the turbine wheel.
The addition of a wastegate to the standard low cost fixed turbine stage adds a cost factor of approximately 16% to that of the fixed turbocharger.
The next level of sophistication in boost control of turbochargers is the VTG (the general term for variable turbine geometry). Some of these turbochargers have rotating vanes and some have sliding sections or rings. Some titles for these devices are: variable turbine geometry (VTG); variable geometry turbine (VGT); variable nozzle turbine (VNT); or simply variable geometry (VG).
VTG turbochargers utilize adjustable guide vanes mounted to rotate between a pair of vane rings and/or one vane ring and a nozzle wall. These vanes are adjusted to control the exhaust gas backpressure and the turbocharger speed by modulating the exhaust gas flow to the turbine wheel. In many configurations the shaft on which the vane rotates is mechanically connected to a vane arm situated above the upper vane ring. The vanes can be rotatably driven by forks engaged in an adjusting ring. In many configurations, the forks on the ends of the vane arms drive independently rotatable “slide blocks” to minimize friction in the system and to deal with the inevitable distortion and corrosion in the turbine housing, and thus the linkages. The adjusting ring must be allowed to rotate circumferentially with minimal friction, and must be aligned radially so that it remains concentric with the upper and lower vane rings (with the vane rings bracketing the vanes; “upper” being closer to the center housing, “lower” being closer to the turbine housing), and axially so that the blocks mounted to the vane ring remain in contact with the vane arms.
FIGS. 3A and 3B show a configuration in which the adjusting ring (33) is supported by ramparts (35) on the vane arms (34). A large block (37) is connected by a shaft to the adjusting ring (33). Circumferential motion of the singular large block (37) about the turbocharger center line (1) causes the adjusting ring (33) to rotate about the turbocharger center line (1). Rotation of the adjusting ring (33) about the turbocharger centerline (1) causes the multiple small blocks (38) to rotate about the turbocharger center line (1) while each of the blocks also rotate about the centerlines (27) of the vane shafts (36). This motion of the small blocks causes the vane arms (34) to rotate about the centerline (27) of the vane shaft (36) and change the angle of attack of the vanes to the exhaust flow. The rotating blocks are designed so that the interface between the block cheeks and the fork cheeks is predominantly sliding friction over the entire area of one cheek of the rotating block. This design provides uniform load distribution, which reduces wear and provides greater life than in line contact, but conversely raises friction over that of a line contact design.
In the example, depicted in FIGS. 3A and 3B, and discussed above, the adjusting ring (33) is constrained and supported by the axial and radial shapes fabricated on the ramparts (35) of the vane arms (34). In another example, depicted in FIGS. 4A and 4B, the adjusting ring is radially supported and constrained by a set of rollers (28) which are themselves either constrained by the turbine housing or upper vane ring. In this configuration, the vane arms (39) are flat and do not contain the ramparts of the prior example.
Turbine housings experience great temperature flux. The outside of the turbine housing is in contact with air at ambient temperature, while the volute surfaces are “wetted” by (are in flowing contact with) exhaust gases ranging from 740° C. to 1050° C., depending on the fuel used in the engine. The turbine housing also experiences large temperature flux from the inlet (5) to the outlet (6) which makes the snail shape volute want to unwind. If the volute shape is constrained in any manner, then the turbine housing tries to twist. Within the confines of these powerful thermal forces distorting the turbine housing, the vane pack (the assembly from lower vane ring (30) to the large turning block (37)) must be supported and constrained in multiple directions. The items in the vane pack are relatively accurate, so tolerances between items in the vane pack are relatively tight. This tight-tolerance design often causes the components of the vane pack to jam during these large thermal changes. Coupled with the need to use expensive-to-procure, machine, and weld (where required) exotic materials which can both tolerate the temperatures and be wetted by the high-temperature corrosive exhaust gas, the VTG becomes quite a costly option in a fiercely cost-competitive market.
If one considers a wastegated turbo as a baseline for cost, then the cost of a typical VTG, in the same production volume, is from 270% to 300% the cost of the same size, fixed, turbocharger. This disparity is due to a number of pertinent factors from the number of components, the materials of the components, the accuracy required in the manufacture and machining of the components, to the speed, accuracy, and repeatability of the actuator. The chart in FIG. 2 shows the comparative cost for the range of turbochargers from fixed to VTGs. Column “A” represents the benchmark cost of a fixed turbocharger for a given application. Column “B” represents the cost of a wastegated turbocharger for the same application; and column “D” represents the cost of a VTG for the same size and application.
Thus it can be seen that, for both technical reasons and cost drivers, there needs to be a relatively lower cost VTG. The target cost price for such a device needs to be in the range of 145% to 165% that of a simple, fixed, turbocharger.